Preparation of cellulose xanthate solution



Patented May 10, 1938 UNITED STATES PATENT OFFICE George A. Richter,Berlin, N. H., assignor to Brown Company, Berlin, N. H., a corporationof Maine No Drawing.

Application January 10, 1936,

Serial No. 58,539

9 Claims.

This invention relates to a process of preparing cellulose xanthatesolution or viscose syrup such as is satisfactory for conversion intoartificial silk, films, or kindred ultimate products by the usualpractices. It deals more especially with a process of preparingcellulose xanthate solution involving an admixture of the various rawmaterials chemically required for the preparation of such solution sothat the finished solution can 10 be realized from essentially only onecontinued mixing and reacting operation performed in a relatively shortperiod of time in a single mixing and reacting vessel.

An object of the present invention is to enable the preparation ofcellulose xanthate solution by such a one-step xanthating operation fromcellulose fiber in an advantageous and economical physical form. Anotherobject thereof is the realization of cellulose xanthate solutionpossessed not only of the appropriate composition, including celluloseand alkali concentration, for conversion into such ultimate products asartificial silk and films, but characterized by such desired andimportant qualities as substantial clarity and freedom even from gelledparticles invisible to the naked eye and without noticeable effect onthe clarity of solution.

In accordance with the present invention, sheets of suitable cellulosepulp, such as the sheets or boards of wood pulp heretofore used inviscose preparation, are initially reduced to small pieces or chips andthe chips then placed in a mixing and reacting vessel adapted for theonestep xanthating operation. While not limited thereto, the mixing andreacting apparatus may advantageously be that disclosed in applicationSerial No. 53,689, filed December 10, 1935, by Orton B. Brown. Generallyspeaking, the apparatus of that patent application, which consists ofjacketed intercommunicating mixing compartments arranged in tandem toafford an elongated reacting vessel, is designed to be revolved about anaxis perpendicular to its long axis either simultaneously with theoperation of the mixing blades in each compartment or independentlythereof. To the vessel containing the pulp chips is added caustic sodasolution preferably but not necessarily of mercerizing activity and thechips are allowed to soak in such solution until they have softened andswollen to such a degree that they may readily be defiberized into afiber suspension or dispersion substantially devoid of fiber aggregatesor clumps. It is preferable to revolve the vessel during the soakingperiod so as to ensure a substantially uniform impregnating,

softening, and swelling action on the chips. Water is then added to thevessel to dilute the causticity and cellulose content of the mixture,for instance, in amount to effect dilution to that desired in thefinished Xanthate solution to be 5 prepared therefrom, whereupon themixers in the several compartments are set in operation and the vesselrevolved until the chips have been completely defiberized and a thickfiber suspension substantially free of fiber aggregates is had. 10 Atthis stage of the operation, liquid carbon bisulphide in amountcalculated to efiect the desired xanthating reaction is added to thevessel and the vessel revolved and its mixers operated until the desiredsubstantially clear xanthate so- 15 lution results. It is usuallypreferable to add dilutlng water to the ingredients prior to theirxanthation in amount insufficient to produce a finished xanthatesolution of the desired dilution, in which case the dilution of thexanthate solution to the desired endpoint is adjusted after xanthationis substantially complete.

The process of the present invention makes possible the use of wood pulpor equivalent cellulose fiber in the form of the relatively dense sheetsor boards heretofore supplied to the viscose-making industry. It iseconomical to put up wood pulp in sheet form at a pulp mill; and suchform of pulp may also be economically stored and shipped. The cost ofcutting the sheets into 30 small pieces or chips, for instance, intochips about one-half inch square, is low. Such chips are of a densitysuch that they occupy a small volume per unit of weight, as compared tofiuffed or shredded cellulose fiber. Thus, about 10 to 15 pounds ofchips of about /2 square cut from the usual wood pulp sheets supplied tothe viscosemaking industry occupies only about 1 cubic foot. Indeed, thedensity or compactness of such chips is so great that considerabledifficulty would be 40 experienced if one had to rely merely upon mixingwith water or dilute caustic soda solution to accomplish a substantiallycomplete defiberization thereof. By preliminarily soaking and swellingthe chips in strong caustic soda solution, 45 however, not only isdisintegration of the chips greatly facilitated so that a fibersuspension substantially devoid of fiber aggregates may be easilyattained, but the dispersed or suspended fiber units are evidentlyactivated for the xanthating 50 reaction, insomuch that the xanthatesolution prepared therefrom is substantially devoid of evenmicroscopically fine undissolved or gelatinous particles. Again, byusing the cellulose in the form of chips, it is possible to wet and soak55 the cellulose substantially uniformly and to activate it with aconcentrated alkali solution in small volume calculated, upon meredilution with water, to yield a finished xanthate solution of thedesired causticity and cellulose concentration. It is thus possible toavoid the use of excess concentrated alkali solution such as wouldnecessitate such extra steps as draining, squeezing, or centrifugingexcess solution from the cellulose.

A specific procedure embodying the present invention may be performed sofar as concerns proportionality of the various ingredients, includingcellulose pulp, caustic soda, water, and carbon bisulphide, to accordwith the disclosure of my patent application Serial No. 37,043 filedAugust 20, 1935. Preferably, but not necessarily, the cellulose pulpherein employed has the low solution viscosity of the pulp specified inthat application. As already indicated, however, the pulp is used insheet form and the sheets are cut into chips of say, about square. Afterthe chips have been added to the mixing and reacting vessel, a causticsoda solution of say, at least about 18% strength, is added to the chipsin volume calculated to yield a finished cellulose xanthate solution ofa cellulose and caustic soda content each falling Within the range of 6%to 10%. The mixture of chips and solution is permitted to stand forabout 20 minutes to 3 hours, during which period the chips become quitesoft and swollen as a result of the soaking of the solution into andthroughout the chips and the attendant mercerizing and swelling actionon the fibers. The mixture may then be advantageously tumbled for ashort while when the reacting vessel is of the type herein'beforementioned. The soaked chips may then be diluted with water in amountcalculated to yield a mixture of the desired fiuidity, for instance, oneof 10% caustic soda content, based on the weight of the mixture, or toyield the desired finished xanthate solution, whereupon the mixingblades of the vessel may be put in motion and the mixture simultaneouslytumbled until the chips have been resolved into the ultimate orindividual fibers to yield a salvelike suspension substantially devoidof fiber aggregates. To this suspension is added liquid carbonbisulphide in the amount necessary for the xanthating reaction and themixed ingredients subjected to the mixing and tumbling treatmentconducive to the formation of the desired finished xanthate solution ina relatively short period of time. Once the xanthate solution has beenprepared, it may be diluted to the desired end-point.

Thus, it is usually preferable to xanthate the fiber while mixed with orsuspended in caustic soda solution in such proportions that the mixturecontains about 10% each of caustic soda and fiber, based on the entiremixture, and, after xanthation is completed, to adjust the dilution ofthe xanthate solution to the desired end-point, for instance, to addwater in amount to dilute each the caustic soda and cellulose content toabout '7 to 7.5%, based on the weight of the xanthate solution, whichlatter concentration is that usually desired in the spinning ofartificial silk. It might be remarked that, excepting the use of thepulp in chip form and the preliminary soaking of the chips in causticsoda solution of preferably mercerizing activity, the process of thepresent invention may be carried out substantially in the same way asdescribed in my application Serial No. 37,043.

In some instances, the preliminary soaking of the chips with causticsoda solution may take place at low temperatures, for instance, at aboutC., as by circulating suitable refrigerant through the jacket of themixing and reacting vessel. While such low temperature soaking may beeffected with caustic soda solution of 15 to 18% or even greaterstrength to increase the mercerizing effect on the chips, yet thecaustic soda solution might be considerably more dilute, for instance,be of such causticity as to require substantially no further dilution toyield a finished xanthate solution of a cellulose and caustic sodacontent each falling within the range of about 7 to 10%. Thus, soakingat temperatures of about 0 C. or lower means that the desiredmercerizing activity on the chips can be realized with caustic sodasolutions of '7 to 10% strength. Of course, once the desired softening,swelling, and mercerizing action has been effected on the chips, themixture may be raised to about to C. preparatory to the defiberizationof the chips. However, it is possible in accordance with the presentinvention to use caustic soda solutions of less than mercerizingcausticity at ordinary temperatures, that is, of less than about 18%strength, and to carry out the soaking of the chips in such solutions atnormal or non-mercerizing temperatures. Moreover, the preliminarysoaking action may be effected on the chips at temperatures higher than20 C., particularly when caustic soda solutions of greater than 18%strength are employed, although solutions of lower strength might alsobe employed in such case. Thus, more rapid penetration of the chips bythe solution is had when the solution is at elevated temperature, say,to C.; and there is less tendency at such temperature for the chips topulp up or release individual fibers or fiber fragments. An elevatedtemperature soaking of the chips also results in some lowering of thesolution viscosity of the cellulose xanthate prepared from the chips sothat the operator may, through control of the temperature of the soakingsolution, produce a finished xanthate solution of particular viscositywithin certain limits, should such control be desired. After the hightemperature soaking, the

soaked mass of chips is preferably cooled to 20 0 C., disintegrated orpulped up, and the resulting fiber suspension xanthated at suchtemperature or other suitable temperature, say, 15 C., or thedisintegration of the soaked mass of chips may first be effected and theresulting fiber suspension cooled to 20 C. preparatory to xanthation atsuch temperature or other suitable temperature.

An important advantage of the present process is that it enables the useof all kinds of cellulose 1;

pulps as raw materials, including pulps derived from birch and otherhardwoods. In the usual Viscose-making process, the use of sulphite orother wood pulps derived from hardwoods generally leads to viscose syrupcontaining myriad fiber fragments or so-called tails, these representingcellulose residue that has escaped the xanthating action of the carbonbisulphide. Such viscose syrup cannot be filtered properly and isconsidered unsatisfactory for conversion into artificial silk or Theinitial step of soaking the pulp chips might, in accordance with thepresent invention, be done by placing them in wire cloth baskets anddipping the baskets into the caustic soda solution of mercerizing ornon-mercerizing activity. Thus, a succession of baskets charged with thechips may be progressively run through the solution to eifect a thoroughsoaking of the chips. Or the chips might be sprayed with the solution asthey are being carried by a foraminous conveyor. Pulp chips treated inthis manner might be permitted to remain soaked with the solution ofmercerizing activity for a sufiiciently long period of time to beconditioned for the subsequent treatments of the present invention. Aspreviously indicated, however, it may be preferable to carry out thesoaking in the same vessel in which xanthation of the pulp is to beeffected and to use soaking solution in amount to avoid the need ofdraining, squeezing, centrifuging, or similar extra step. Aside from theextra equipment necessary when soaking of the chips is effected outsideof the xanthating vessel, a basket-dipping of the chips into the soakingsolution or the spraying of the chips with the solution does not readilylend itself to control of the amount of solution associated with thechips, in consequence of which it is necessary to associate excesssolution with the chips and then to employ the additional step ofdraining, squeezing, or centrifuging and the equipment incidentalthereto.

There are instances, however, when it may be advantageous to soak thechips in caustic soda solution containing caustic soda in amount inexcess of that required for realizing from the mix ture of chips andsoaking solution a xanthate solution of particular desired cellulose andcaustic soda content. Thus, it is possible in such instances to effectpurification of the pulp chips upon removing therefrom, as by drainage,an

amount of solution containing the excess caustic soda. In suchinstances, the chips may be soaked in a large excess of caustic sodasolution of appropriate concentration and, after the chips have beensubstantially uniformly penetrated by the solution, they may be draineduntil they contain only the mount of caustic soda necessary for thepreparation of a finished xanthate solution of the desired cellulose andcaustic soda content. The amount of solution drained from the chips maybe controlled by titrating the chips and/or solution squeezed from thechips and/or by measuring the volume of solution being drained from thechips as drainage proceeds so as to ensure in the drained chips thealkali content requisite for the xanthate solution to be preparedtherefrom. It is found that the chips do not lose their physical formduring soaking and. drainage, wherefore they may be readily transferred,after drainage, to the xanthating vessel, assuming that preliminarysoaking is performed in separate equipment. When the pulp subjected tosuch preliminary soaking contains non-alpha cellulose impurities capableof being acted upon and dissolved by the soaking solution, the excesssolution removed from the chips contains part of the dissolved nonalphacellulose impurities, in consequence of which the drained chipssubjected to xanthation are of greater purity than the chips used as thestarting material and hence yield xanthate solutions of improvedquality. When working with excess soaking solution, it is possible touse for the purpose of the present invention caustic soda solution ofless than mercerizing strength, so

long as it contains a sufficient concentration of caustic soda toassociate with the chips, after appropriate drainage, the amount ofcaustic soda necessary for the desired finished xanthate solution.Depending upon the strength of the solution used in the soaking stepand/or upon the amount of soaking solution drained from the soaked chipsand/or upon the composition of finished xanthate solution desired,diluting water may or may not be added during the disintegration of thesoaked chips effected preparatory to xanthation. It might be noted thatthe use of pulp in chip form is of advantage even with solution ofnon-mercerizing strength or activity, as a mass of chips is readily anduniformly penetrated by such solution, and, after the soaking period,excess solution is readily drained therefrom by reason of themultitudinous comparatively large interstices afforded in the body ofsuch a mass. On the other hand, pulp in fluffed or shredded form doesnot permit of these advantages or the advantage of easy transfer to aXanthating vessel without loss of fibers and fiber fragments. Moreover,pulp in chip form once having undergone preliminary soakingdisintegrates readily in a xanthating vessel or mixer into the ultimateor individual fibers of which the chips are composed toyield asalve-like suspension substantially devoid of fiber aggregates and hencereadily xanthatable into a finished xanthate solution of the desiredquality. These results cannot be had through the processing of largesheets of pulp such as enter into the usual viscose-making process.

The sheets or pulpboard employed pursuant to the present invention mayvary in its density or compactness. The pulpboard ordinarily used isproduced from substantially unbeaten wood pulp and may have acompactness or density varying from about to 120 and a thickness rangingfrom about .030 to .060 inch. However, it is possible to use sheetswhose compactness and thickness are considerably greater or less thanthese values. The pieces or chips into which the sheets are cut may varyin size, but it is preferable that they have an area no greater thanabout one square inch. If desired, the sheets may contain residualmoisture, particularly when moisture facilitates their cutting intochips.

The sheet compactness values herein given are obtained by dividing thebasis Weight of the sheet in pounds by its thickness in inches andmultiplying by the factor 10 The basis weight values herein givensignify the weight in pounds of 480 sheets Whose dimensions are 24 x 36inches, that is, the weight of 28-80 square feet of the sheet material.

I claim:-

1. A process of preparing cellulose xanthate solution from sheets ofcellulose fiber, which comprises cutting the sheets into chipsconsisting.

of fiber aggregates, soaking the chips with caustic soda solution ofmercerizing activity to soften and swell them and to associate With thefiber the caustic soda desired in the finished xanthate solution,disintegrating the softened chips to form a fiber suspension in causticsoda solution substantially devoid of fiber aggregates, said lastnamedsolution containing caustic soda solution furnished by the solution ofmercerizing activity and being in amount and in concentration calculatedto yield a finished xanthate solution of a cellulose and caustic sodacontent each falling Within the range of 6% to 10%, and adding carbonbisulphide to said suspension and mixing the ingredients for asufiicient period of time to cause them to react substantially tocompletion to yield the desired cellulose xanthate solution.

2. A process of preparing cellulose xanthate solution from sheets ofcellulose fiber, which comprises cutting the sheets into chipsconsisting of fiber aggregates, soaking the chips with caustic sodasolution of at least about 18% strength to soften and swell them and toassociate with the fiber caustic soda in amount desired in the finishedxanthate solution, diluting the caustic soda solution associated withthe fiber with water in amount calculated to yield a finished xanthatesolution of a cellulose and caustic soda content each falling within therange of 6% to 10%, disintegrating the softened chips to form a fibersuspension in such diluted solution substantially devoid of fiberaggregates, and adding carbon bisulphide to said suspension and mixingthe ingredients for a sumcient period of time to cause them to reactsubstantially to completion to yield the desired cellulose xanthatesolution.

3. A process of preparing cellulose xanthate solution, which comprisesforming from substantially unbeaten wood pulp a pulpboard having athickness of about 0.030 to 0.060 inch and a compactness of about to120, cutting the pulpboard into chips of an area not greater than aboutone square inch, soaking the chips with caustic soda solution of atleast about 18% strength to soften and swell them and to associate withthe fiber caustic soda in amount desired in the finished xanthatesolution, diluting the caustic soda solution associated with the fiberwith water in amount calculated to yield a finished xanthate solution ofa cellulose and caustic soda content each falling within the range of 6%to 10%, disintegrating the softened chips to form a fiber suspension insuch diluted solution substantially devoid of fiber aggregates, andadding carbon bisulphide to said suspension and mixing the ingredientsfor a sufficient period of time to cause them to react substantially tocompletion to yield the desired cellulose xanthate solution.

4. A process of preparing cellulose xanthate solution from sheets ofcellulose fiber, which comprises cutting the sheets into chipsconsisting of fiber aggregates, soaking the chips with caustic sodasolution to soften them, the amount and concentration of solutionemployed being such that its caustic soda content is in excess of thatcalculated to yield a finished xanthate solution of a cellulose andcaustic soda content each falling within the range of 6% to 10%,removing from the soaked chips substantially only an amount of causticsoda solution containing such excess caustic soda, disintegrating thesoftened chips in the presence of the caustic soda solution retainedthereby and added diluting water to form a fiber suspension in causticsoda solution substantially devoid of fiber aggregates and containingcaustic soda in amount and in concentration calculated to yield afinished xanthate solution of a cellulose and caustic soda content eachfalling within the range of 6% to 10%, and adding carbon bisulphide tosaid suspension and mixing the ingredients for a sufiicient period oftime to cause them to react substantially to completion to yield thedesired cellulose xanthate solution.

5. A process of preparing cellulose xanthate solution from sheets ofcellulose fiber, which comprises cutting the sheets into chipsconsisting of fiber aggregates, soaking the chips with causticsoda-solution of mercerizing activity to soften and swell them, theamount and concentration of soaking solution employed being such thatits caustic soda content is in excess of that calculated to yield afinished xanthate solution of a cellulose and caustic soda content eachfalling within the range of 6% to 10%, re moving from the soaked chipssubstantially only an amount of caustic soda solution containing suchexcess caustic soda, disintegrating the softened chips in the presenceof the caustic soda solution retained thereby and added diluting waterto form a fiber suspension in caustic soda solution substantially devoidof fiber aggregates and containing caustic soda in amount and inconcentration calculated to yield a finished xanthate solution of acellulose and caustic soda content each falling within the range of 6%to 10%, and adding carbon bisulphide to said suspension and mixing theingredients for a sufficient period of time to cause them to reactsubstantially to completion to yield the desired cellulose xanthatesolution.

6. A process of preparing cellulose xanthate solution from sheets ofcellulose fiber, which comprises cutting the sheets into chipsconsisting of fiber aggregates, soaking the chips with caustic sodasolution of at least about 18% strength to soften and swell them, theamount of soaking solution employed being such that its caustic sodacontent is in excess of that calculated to yield a finished xanthatesolution of a cellulose and caustic soda content each falling within therange of 6% to 10%, removing from the soaked chips substantially only anamount of solution containing said excess caustic soda, disintegratingthe softened chips in the presence of the caustic soda solution retainedthereby and added diluting water to form a fiber suspension in causticsoda solution substantially devoid of fiber aggregates and containingcaustic soda in amount and in concentration calculated to yield afinished xanthate solution of a cellulose and caustic soda content eachfalling within the range of 6% to 10%, and adding carbon bisulphide tosaid suspension and mixing the ingredients for a sufiicient period oftime to cause them to react substantially to completion to yield thedesired cellulose xanthate solution.

'7. A process of preparing cellulose xanthate from sheets of cellulosefiber, which comprises cutting the sheets into chips consisting of fiberaggregates and having an area of about one-quarter to about one squareinch, soaking the chips with caustic soda solution to soften them and toassociate with the fiber sufficient caustic soda to permit itsconversion into cellulose xanthate, disintegrating the softened chips toform a fiber suspension substantially devoid of fiber aggregates incaustic soda solution containing caustic soda furnished by said soakingsolution, and adding carbon bisulphide to the suspension and mixing theingredients to cause them to react to yield cellulose xanthate.

8. A process of preparing cellulose xanthate from sheets of cellulosefiber, which comprises cutting the sheets into chips consisting of fiberaggregates and having an area of about 0nequarter to about one squareinch, soaking the chips with caustic soda solution of mercerizingactivity to soften and swell them and to associate with the fibersufficient caustic soda to permit its conversion into cellulosexanthate, disintegrating the softened chips to form a fiber suspensionsubstantially devoid of fiber aggregates in caustic soda solutioncontaining caustic soda furnished by said soaking solution, and addingcarbon bisulphide to said suspension and mixing the ingredients to causethem to react to yield cellulose xanthate.

9. A process of preparing cellulose xanthate from sheets of cellulosefiber, which comprises cutting the sheets into chips consisting of fiberaggregates and having an area of about one-quarter to about one squareinch, soaking the chips with caustic soda solution of at least about 18%strength to soften and swell them and to associate with the fibersuflicient caustic soda to permit its conversion into cellulosexanthate, disintegrating the softened chips to form a fiber suspensionsubstantially devoid of fiber aggregates in caustic soda solutioncontaining caustic soda furnished by said soaking solution, and addingcarbon bisulphide to said suspension and mixing the ingredients to causethem to react to yield cellulose xanthate.

GEORGE A. RICHTER.

